What is your reason for starting mining?
The reason to start mining rather than just buying bitcoins should be because you have considered your interest to be interested in mining and also other factors that will make mining profitable to know if it is something you can do. If you consider factors like cheap affordable electricity, security etc and see that it's a check for all those factors, then you can start mining.
How do you define whether a supplier is legit?
Do not buy from a new supplier on the market, buy from suppliers who have been in the business and were recommended.